The invention relates to a method for producing a zinc oxide layer and to a zinc oxide layer.
Polycrystalline sputtered zinc oxide (ZnO) layers are used as transparent contact layers in optoelectronic components, for example in solar cells. ZnO is an inexpensive alternative to other transparent contact materials such as indium tin oxide (ITO).
It is known that zinc oxide can be etched in hydrochloric acid. The surface of the layer is laterally structured by way of wet-chemical etching of the zinc oxide layer. In an anisotropic etching process, this creates a texture having structures in the nanometer or micrometer range.
Polycrystalline sputtered zinc oxide layers are used in thin-film solar cells as the front contact layers, and as what is known as an intermediate reflector, or, in combination with a reflector, as the back contact layer.
Polycrystalline sputtered zinc oxide layers, which are subsequently structured by way of a wet-chemical process, are used as transparent contact layers in thin-film solar cells. The rms roughness of sputtered zinc oxide layers is usually less than 5 to 15 nanometers. A ZnO layer can be etched at high speed in diluted hydrochloric acid. This increases the rms roughness of the zinc oxide surface, creating craters required for scattering light.
The diameters and depths of the craters can be varied by varying the etching time. In this process, the number and/or density thereof change only insignificantly. The structures also cannot be changed significantly by means of the pH value. As the pH value increases in the acid range, the etching time increases until a defined crater depth has been reached.
In terms of the size and density of the structures, the texture resulting after the etching process actually depends on the ZnO layer properties before etching. These properties, in turn, are influenced by various parameters during the production of the ZnO layers by means of sputtering. The influencing parameters include (a) the substrate and the pretreatment thereof, (b) the sputtering conditions, such as the substrate temperature, the discharge power, the sputtering pressure, the gas composition and the doping. The shapes of the structures and craters that develop during etching can be influenced within narrow limits by the etching solution and the duration. The selection of the etching duration or etching medium changes the numbers and sizes of the craters only insignificantly. Essentially three types of etched surfaces are distinguished when etching in acids:    Type 1: The type 1 surface topography is microscopically rough and has sharp-edged surface structures with crosswise dimensions of approximately 300 nm and very steep flanks. FIG. 1a shows the prior art, which features such a rough surface having an almost Gaussian statistical distribution of the elevations. Typical opening angles of the structures are 40° to 80°. The crosswise dimensions of the craters are less than 300 nm. The topography of the surface after etching depends on the etching time. For a zinc oxide layer having a thickness of approximately 800 nm, of which approximately 150 nm is removed by means of wet-chemical ablation in hydrochloric acid, the rms roughness is approximately 50 to 120 nm and the crosswise correlation length is between 100 and 300 nm.    Type 2: With the type 2 surface topography, the surface is covered almost uniformly with large craters (FIG. 1b). The crosswise crater diameters are 0.5 μm to 3 μm and the crater depths range between 150 nm and 400 nm. The craters typically have opening angles of approximately 120° to 135°. The topography of the surface after etching depends on the etching time. For a layer having a thickness of approximately 800 nm, of which approximately 150 nm is removed by means of wet-chemical ablation in hydrochloric acid, the rms roughness is between 100 and 180 nm, and typically approximately 135 nm, and the crosswise correlation length is between 400 and 1000 nm.    Type 3: The type 3 surface topography likewise has large craters (FIG. 1c), which are surrounded by relatively planar areas. The planar area comprises only small flat craters having a depth of up to approximately 100 nm. Some isolated large craters having a crosswise expanse of up to 3 μm extend to the substrate, whereby a plateau is formed there. The topography of the surface after etching depends on the etching time. For a layer having a thickness of approximately 800 nm, of which approximately 150 nm is removed by means of wet-chemical ablation in hydrochloric acid, the rms roughness is less than 100, typically around 20 to 50 nm, and the crosswise correlation length is between 250 and 800 nm.
For types 2 and 3 in FIGS. 1b and 1c, the height distribution is generally highly asymmetrical. This is due to surface structures forming in a crater-shaped manner as a result of the etching process. Typical opening angles of the structures are 120° to 140°. The rms roughness is highly dependent on the etching duration.
Type 2 (FIG. 1b) differs from type 3 (FIG. 1c) in the number of craters per unit of surface area. Type 2 has a greater crater density. A layer having a type 2 surface thus has higher rms roughness as compared to type 3 with the same layer ablation.
Because the craters make a positive contribution to light scattering, a type-2 surface is better suited for thin-film solar cells than the surface according to type 3 or type 1.
The surface texture of the zinc oxide layer that develops from the etching process causes the light impinging on a solar cell to be scattered. Upon impingement, and after passing through the zinc oxide layer, the light is scattered in the absorber layer and ideally reflected multiple times inside the cell (“light-trapping” effect), resulting in improved cell efficiency.
Crater structures having a diameter of approximately 0.5 to 3 μm, which are uniformly dispersed over the surface of the layer, (type 2) have proven to be advantageous for light scattering in solar cells. The relationship is known from Berginsky et al. (Berginsky M., Hüpkes J., Schulte M., Schöpe G., Stiebig H., Rech B., Wuttig M. (2007) The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells. Journal of Applied Physics 101, 074903-1-11).
The zinc oxide layers shown in FIGS. 1a-c are created after etching for 30 seconds in hydrochloric acid (0.5 wt. % (percent by weight)). These layers are usually produced by sputtering. Selection of the sputtering parameters determines the type of surface topography that is obtained after etching. With deposition conditions such as a 2 kW discharge power, a 200 sccm argon gas flow and a 10 μbar deposition pressure, a surface of type 2 is obtained after etching in hydrochloric acid. For this purpose, non-reactive sputtering of ceramic zinc oxide tube targets comprising aluminum oxide doping of 0.5 wt. % at a medium frequency (MF) excitation of 40 kHz is employed. In contrast, with deposition conditions such as a 14 kW discharge power, a 200 sccm argon gas flow and a 20 μbar deposition pressure, a surface similar to type 3 is obtained after etching in hydrochloric acid.
The influence of the target doping of planar ceramic ZnO:Al2O3 targets and that of the substrate temperature on the surfaces created after etching were also analyzed. For example, a radio-frequency (rf) sputtering process at low power was employed. The power density (2 W/cm2) and argon flow (100 sccm) process parameters were kept constant for all the experiments. After etching, type-1 surfaces are obtained, preferably with low substrate temperatures and low target doping levels, for example with 0.2% wt % Al2O3 doping at 170° C. In contrast, after etching, surfaces according to type 3 are obtained with high substrate temperatures and high target doping levels, for example with 2% wt % Al2O3 doping at 460° C. After etching, type-2 surfaces are obtained in a narrow process window with respect to the temperature between type 1 and type 3 with low to medium temperatures, for example with 1 wt % Al2O3 doping and a substrate temperature of approximately 300° C. As the target doping decreases, the process window in which the zinc oxide layer after etching can be etched in a surface of type 2 becomes even narrower.
In addition, the influence of the sputtering pressure and of the substrate temperature on the sputtered zinc oxide layers was analyzed. After etching in 0.5 wt % HCl, a type-1 surface develops with a sputtering pressure of 20 μbar and a substrate temperature of 270° C. A type-2 surface develops after etching in 0.5 wt % with a sputtering pressure of 2.7 μbar and a substrate temperature of 270° C.
Typically, 0.5 wt % (0.137 N) is used for etching the zinc oxide layers. Different etching solutions such as sulfuric acid, phosphoric acid or citric acid are also able to etch zinc oxide. In principle, topographies similar to those obtained with etching in hydrochloric acid develop. The resulting surfaces exhibit similar characteristics to those described as types 1 to 3. Crater-shaped surfaces also develop, with the etching properties being essentially defined by the layer properties, which are determined by the selection of the sputtering parameters.
The location at which the crater develops during the etching process of the ZnO layer is apparently already influenced by the production conditions of the zinc oxide layer as an inherent layer property. Because these parameters, and in particular the substrate temperature during sputtering, cannot be satisfactorily uniformly adjusted on a large industrial scale, even with great technical expenditure, the etching methods according to the prior art have the disadvantage of creating large amounts of undesirable scrap or subregions of a ZnO layer with type-1 and type-3 layers. This is because, in the further production process, the etching medium defines only the shape of the etching structures, but not the surface density thereof. Thus, type-2 layers develop in a partially random manner, and without the required degree of homogeneity and reproducibility.
Using a alkaline solution as the etching medium also does not offer a solution in this respect. Alkaline solutions result in deep holes in the zinc oxide layers, as is shown in FIG. 1d. Physical ablation results in a large number of shallow depressions. In both cases, the density of the points of attack is defined by the layer properties. These layers are not suitable for light scattering in solar cells.
In addition to the lack of reproducibility, the previously known etching methods have other disadvantages, notably:    1. It is not possible to cost-effectively produce double structures, which is to say craters in craters having good light scattering properties for zinc oxide layers on large-surface-area substrates (>0.1 m2). Double structures are desirable because they result in improved light scattering in SnO2 layers.    2. The sizes and numbers of the craters in the etched zinc oxide layers can be controlled only to a very limited extent by the sputtering process or the etching process in various liquid media, such as acids or alkaline solutions.    3. The process windows for producing the zinc oxide layer in which suitable surface structures are created by the etching process are narrow. Highly conductive zinc oxide layers produced at a high deposition rate usually exhibit only low crater density after etching. This has the disadvantage of resulting in the photovoltaic solar modules produced on these zinc oxide layers having low efficiencies.